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Molecular Epidemiology of Macrolide-Resistant Isolates of Streptococcus pneumoniae Collected from Blood and Respiratory Specimens in Norway

University Hospital of North Norway (UNN) and Department of Microbiology and Virology, Institute of Medical Biology, Faculty of Medicine, University of Tromsø, Tromsø,¹ Division of Infectious Disease Control, Norwegian Institute of Public Health,² Institute of Oral Biology, University of Oslo, Oslo, Norway³

Received 21 August 2004/ Returned for modification 6 December 2004/ Accepted 7 January 2005

Norway has a low prevalence of antimicrobial resistance, including macrolide-resistant Streptococcus pneumoniae (MRSP). In a nationwide surveillance program, a total of 2,200 S. pneumoniae isolates were collected from blood cultures and respiratory tract specimens. Macrolide resistance was detected in 2.7%. M-type macrolide resistance was found in 60% of resistant isolates, and these were mainly mef(A)-positive, serotype-14 invasive isolates. The erm(B)-encoded macrolide-lincosamide-streptogramin B (MLS_B) type dominated among the noninvasive isolates. One strain had an A2058G mutation in the 23S rRNA gene. Coresistance to other antibiotics was seen in 96% of the MLS_B-type isolates, whereas 92% of the M-type isolates were susceptible to other commonly used antimicrobial agents. Serotypes 14, 6B, and 19F accounted for 84% of the macrolide-resistant isolates, with serotype 14 alone accounting for 67% of the invasive isolates. A total of 29 different sequence types (STs) were detected by multilocus sequence typing. Twelve STs were previously reported international resistant clones, and 75% of the macrolide-resistant isolates had STs identical or closely related to these clones. Eleven isolates displayed 10 novel STs, and 7/11 of these "Norwegian strains" coexpressed MLS_B and tetracycline resistance, indicating the presence of Tn1545. The invasive serotype-14 isolates were all classified as ST9 or single-locus variants of this clone. ST9 is a mef-positive M-type clone, commonly known as England¹⁴-9, reported from several European countries. These observations suggest that the import of major international MRSP clones and the local spread of Tn1545 are the major mechanisms involved in the evolution and dissemination of MRSP in Norway.

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